The invention relates to forming a ceramic coating on a tape substrate. More particularly, the invention relates to an apparatus and method for converting a metal deposit on a tape substrate to a ceramic, such as an oxide or a nitride. Even more particularly, the invention relates to an apparatus and method of forming a high temperature superconducting oxide on a tape substrate.
The formation of ceramic materials such as oxides, nitrides, and the like on a tape substrate is of particular interest, particularly in the filed of power generation and distribution. High temperature superconducting (HTS) oxides fall within this class of materials, and are being incorporated into silver-based tapes for use in motors, generators, and power transmission lines.
Several methods are currently being used to deposit HTS coatings. These include metallorganic-based liquid deposition techniques; evaporative techniques, including thermal (resistance or induction) and electron beam evaporation; vaporization by pulsed lasers; and metallorganic chemical vapor deposition (MOCVD). Of these methods, evaporative methods are particularly desirable because they offer a potentially high throughput and, therefore, scalable technology with broad area deposition and high rate of film formation.
One inherent drawback in evaporation, relates to the phase stability of the HTS material itself. To obtain the desired superconducting layered perovskite structure having a high degree of crystalline texture, an atmosphere with sufficiently high oxygen partial pressure (PO2) is needed. A high oxygen partial pressure, however, is not compatible with evaporative processes.
One approach to this problem is to expose the substrate alternately to a high PO2 region (referred to as the “oxygen pocket”) where HTS stability is established, and a low PO2 region where evaporation is used to deposit the metal constituents of the HTS material. This may be accomplished by first depositing the metal constituents and then transferring the coated substrate through a separate furnace for oxygen annealing. In another variation, the substrate is translated back and forth between the oxygen pocket and the low PO2 region. However, the latter method is unable to coat substrates—such as tapes, wire, and ribbons—having one dimension (length) that is many times greater that the other two dimensions and much larger than the apparatus itself.
To overcome the dimensional problem, a tape substrate is helically wound around a single mandrel and sections of the resulting spiral arrangement are alternately passed through the oxygen pocket and the low PO2 region. While this technique allows for a greater length of tape substrate to be coated, the length of the tape substrate is essentially limited by the size of the mandrel. Therefore, a very large mandrel is necessary to manufacture lengths of tape needed for many power generation and motor applications. In addition, the tape tends to creep and set because it is helically wound around the mandrel, making the tape unacceptable for some applications.
Another variation employs spiraling the tape around multiple reels or rollers to provide a continuous feed of tape in conjunction with a movable oxygen pocket. When a portion of the tape substrate is the vicinity of the oxygen pocket, the tape is not necessarily in thermal equilibrium, as the radiative properties of the tape change during deposition.
The length of HTS tape that can currently be manufactured in limited in part by the dimensions of the manufacturing equipment. In addition, it is difficult to maintain thermal equilibrium of the tape substrate during the oxygenation process. Therefore, what is needed in an apparatus that allows a high temperature superconductor to be deposited on long, continuous lengths of tape substrate. What is also needed is an apparatus that is capable of maintaining the tape substrate in thermal equilibrium during the deposition process.